1. Field of the Invention
The present invention relates to magnetic recording/reproducing apparatus, and more particularly to magnetic recording/reproducing apparatus for recording/reproducing broad-band video signals such as HDTV (High Definition Television) signals.
2. Description of the Background Art
Conventionally, there is no relationship between the phase of a video signal and the phase of a carrier FM-modulated with the video signal (hereinafter, referred to as FM carrier) in FM recording/reproducing of a video signal. Accordingly, in reproducing, even a part of the lower sideband of the FM carrier slightly leaks into the band of a FM demodulation video signal and, it damages the picture stability as beat stripes or moire (stripe-like noise due to the beat interference). That is to say, so-called beat stripe drifts occur on a screen. A human being is very sensitive in detecting the beat stripe which produces dynamic distortion. For example, even when the amplitude of a leaking portion of a lower sideband of the FM carrier (a peak-peak value) which is a cause of the beat stripe is about 1/200 of the amplitude (a peak-peak value) of a video signal (0.5%, -46 dB), the beat stripe is detectable. Generally it is said that the DU ratio (Desired-to-Undesired Signal Ratio) of about +35 dB is the allowable threshold permittable as to home VTR products.
Now, referring to the wave form diagram of FIG. 7, the occurrence of the beat stripe due to fluctuation of the difference between the phase of FM carrier and the phase of horizontal synchronizing signal will be described below in detail.
Generally, it is known that when a carrier is FM-modulated with a video signal e(t), as shown by the following expression, an infinite number of sidebands are produced for every angular frequency .omega..sub.p ; EQU e(t).sub.FM =A.sub.N .SIGMA.J.sub.N (m) sin [(.omega..sub.C +N.omega..sub.P)+.alpha.]
In the above expression, J.sub.N (m) indicates the Bessel function of the first kind, .omega..sub.C indicates the angular frequency of a video signal, .alpha. indicates the angular frequency of FM carrier, .alpha. indicates a value obtained by measuring the phase of the FM carrier (frequency) signal with a reference of a leading edge or a trailing edge of the horizontal synchronizing pulse, and N indicates an integer from -.infin. to +.infin..
FIG. 7 (a) shows the reference phase of a rise in the leading edge or the trailing edge of a horizontal synchronizing pulse. FIG. 7 (b) shows video signal. The video signal is always in synchronization with the horizontal synchronizing pulse. The solid line in FIG. 7 (c) indicates a waveform of the FM carrier at the reference phase, or .alpha.=0. The basic wave spectrum of the FM carrier is shown in FIG. 7 (d). The first lower sideband and the first upper sideband of the FM carrier are shown in FIGS. 7 (e) and (f), respectively As clearly seen from FIGS. 7 (d)-(f), the phases of each of the basic wave spectrum, the first lower sideband, and the first upper sideband of the FM carrier are all in synchronization with the reference phase of the leading edge or the trailing edge of the horizontal synchronizing signal. Accordingly, by always maintaining the phase relationship, the phase of the spectrum of the lower sideband which is a cause of the beat stripe occurrence is in synchronization with the horizontal synchronization pulse, resulting in no beat stripes "drifting" on the screen.
On the other hand, if the phase of the FM carrier shifts by 90.degree. (.alpha.=90.degree.), for example, with respect to the reference phase of the horizontal synchronizing pulse as shown by the broken line in FIG. 7 (c), the phase of the spectrum of the first lower sideband and the phase of the spectrum of the first upper sideband also shift by 90.degree. accordingly as shown by the broken lines in FIGS. 7 (d), (e) and (f). As a result, if the spectrum of the first lower sideband invades into the band of the demodulated video signal, due to fluctuation in phase .alpha., the beat stripes appear to drift on the screen. The video signal is always in synchronization with the horizontal synchronizing signal as shown in FIG. 7 (b).
Referring to FIGS. 3 through 6, the method of preventing beat stripes in the FM demodulating system conventionally used in the MUSE (Multiple Sub-Nyquist Sampling Encoding) VTR (Video Tape Recorder) will be described below.
FIG. 3 is a block diagram indicating a schematic structure of an FM modulating/demodulating system of a conventional MUSE.VTR. An inputted MUSE signal, to which a negative pole synchronization signal and a reference burst signal are added in a signal processing circuit 31 and the time base compression is applied, is then FM modulated in FM modulator 32 having an AFC (Automatic Frequency Control) circuit to be recorded in magnetic tape 35 through a recording amplifier 33 and a magnetic head. Various kinds of timing pulses necessary for operations of signal processing circuit 31 and FM modulator 32 are produced by a timing pulse generator 34 on the basis of the MUSE signal.
A MUSE signal, in which positive pole synchronization is introduced, cannot be recorded on magnetic tape as it is (refer to FIG. 4 (a)). In order to make a MUSE signal recordable on magnetic tape, as shown in FIG. 4 (b) for example, the MUSE signal is time-base compressed to 9/10 times in signal processing circuit 31 for every period of the horizontal synchronizing signal. The time-base compressed MUSE signal, in which a negative pole synchronizing signal and a reference burst signal are inserted into a blanking time period (about 2.9 .mu.sec) caused by the time-base compression, is recorded on magnetic tape 35. The arrows in FIGS. 4 (a), (b) indicate phases at which the positive pole synchronization is used. On the other hand, in reproducing, the jitter correction of the reproduced signal is executed in a TBC (Time Base Correct) circuit 43 on the basis of the negative pole synchronization signal and the reference burst signal, and then the time-base expansion process of 10/9 times is applied to the reproduced signal in a time base expansion circuit 44 to reproduce an original MUSE signal.
The band of the MUSE signal provided for use in signal processing circuit 31 is 8.1 MHz, but as a result of the time base compression process of 9/10 times, the required bandwidth becomes 9 MHz. Accordingly, as shown in FIG. 5 (a), when the carrier is FM-modulated with a MUSE signal in which a modulation frequency f.sub.P of 9 MHz is superimposed upon the mid gray level to be recorded on magnetic tape 35, the spectrum of the FM carrier becomes as shown in FIG. 5 (b) in used the process of FM demodulation in reproducing. That is to say, in reproducing, the recorded information is picked up by the magnetic head from magnetic tape 35, which is provided as an input to an equalizer 37 through a head amplifier 36. The spectrum of the FM carrier provided as an output from equalizer 37 is shown in FIG. 5 (b). In the FM modulation parameters, the frequency of the center carrier F.sub.C is set at 16 MHz, the frequency deviation .DELTA.F is set at .+-.4 MHz, the input maximum frequency or the modulating frequency F.sub. P is set at 9 MHz, and the modulation index m: is set at 0.44.
The FM carrier outputted from equalizer 37 is provided to a first low-pass filter 38 as shown in FIG. 3. Although the cut-off frequency of the first low-pass filter 38 is defined by the frequency characteristics of an output of the magnetic head, it is defined as 36 MHz for convenience. As shown in FIG. 5 (b), the components of the second lower sideband of the FM carrier (-2 MHz, the ratio with respect to the carrier, 2.4%) is folded over into the positive frequency range, and if it is demodulated as it is, it comes into the video signal band (9 MHz). Generally, the band of a demodulated video signal (hereinafter, referred to as demodulated video band) is regarded as extending from -9 MHz to +9 MHz.
The higher the order of the sideband of the FM carrier is, the smaller its spectral strength becomes, so that the demodulation process should be used after shifting the frequency of the central carrier F.sub.C as high as possible for preventing the beat interference. In order to implement that, conventionally,
(a) a frequency doubler of the FM carrier before the FM demodulating process, and PA0 (b) a pulse count type demodulator with a doubler function, for example,
are introduced and cascade-connected for quadruple demodulation of the FM carrier.
As shown in FIG. 3 for example, an output of first low-pass filter 38 is provided to doubler 39 as an input, where the frequency of the FM carrier is doubled. By the doubling process by doubler 39, as shown in FIG. 5 (c), the modulation parameters of the FM carrier are converted, that is, the frequency of the center carrier F.sub.C2 is converted into 32 MHz, the frequency deviation .DELTA.F.sub.2 into .+-.8 MHz, and the modulation index m.sub.2 into 0.89, respectively. Accordingly, the second lower side band of the FM carrier is converted into 14 MHz, which is out of the demodulation video band (10 MHz). Accordingly, the beat interference does not occur. However, each component of the third lower sideband (5 MHz, the ratio with respect to the carrier is 1.2%, -38 dB), and the fourth lower sideband (-4 MHz, the ratio with respect to the carrier is 0.15%, -56 dB) of the FM carrier invades the demodulated video band. Therefore, for cutting off each component of the third lower sideband and the fourth lower sideband of the FM carrier, as shown in FIG. 3, a high-pass filter 40 is provided. The cut-off frequency of high-pass filter 40 is 10 MHz or more. An output of high-pass filter 40 is provided to a frequency-doubling FM demodulator 41 of the pulse count type (hereinafter, referred to as a pulse count type FM demodulator) to be demodulated.
The output of the pulse count type FM demodulator 41 includes, besides the demodulated video signal, a doubled FM carrier of the center carrier F.sub.C, the frequency deviation .DELTA.F, and the modulation index m.sub.1, respectively, similarly to the above-mentioned doubler 39. Then, the FM carrier component is provided as an output as it is mixed with demodulated video signal component. Therefore, for eliminating the main FM carrier components and extracting a video signal, a second low-pass filter 42 is provided. That is, as shown in FIG. 3, an output of pulse count type FM demodulator 41 is sent to the second low-pass filter 42. However, a part of the lower sideband of the FM carrier then gets into the demodulation video band, and if the level is higher than an allowable threshold, a picture quality deterioration due to the beat interference occurs.
On the other hand, when the FM carrier is quadruple demodulated by the cascade connection of doubler 39 and pulse count type FM demodulator 41, in the modulation parameters of the FM carrier, the frequency of the main carrier F.sub.C4 is converted into 64 MHz, the frequency deviation .DELTA.F.sub.4 into .+-.16 MHz, and the modulation index m.sub.4 is into about 1.78. Accordingly, what is present in the demodulation video band is the seventh lower sideband of the FM carrier (1 MHz, the ratio with respect to carrier is about 0.01%), and the beat stripes are less than the detectable threshold so as not to be visible (refer to FIG. 5 (e)). When such an ideal operation occurs, even if the frequency of the center carrier F.sub.C of the FM modulation is decreased to 12 MHz, the fifth lower sideband (3 MHz, the ratio with respect to carrier is 0.4%) of the FM carrier which causes a problem in quadruple multiplication demodulation is at around the level of a detectable limit of beat stripes. Therefore, the pulse count type FM demodulator 41 for preventing beat stripes becomes unnecessary.
In an analog multiplier for configuring a doubler, which is of wide-band, and generally available at the present time, the output considerably includes error or spurious components due to leakage from the input FM carrier and the like. That is to say, as shown in FIG. 5 (d), the leak component F.sub.C ' from the input FM carrier generally exists in an output of the doubler 39, which is about 20% of the center carrier component F.sub.C 2 multiplied by doubler 39. In this case, the first lower sideband (7 MHz, the ratio with respect to leak carrier is 21%) of the leak component reaches 4% of the doubled center carrier component, which causes beat stripes.
Next referring to FIG. 6, the conditions of satisfying the demodulation in the pulse count type FM demodulator 41 will be described below. A monomultivibrator (not shown) is triggered at the zero cross point of an input carrier, and the output pulses (pulses shown in FIG. 6 (a)-(c)) are averaged by the second low-pass filter 42 to demodulate a video signal.
The strict condition of satisfying the doubling demodulation in pulse count type FM demodulator 41 is that the inputted FM carrier includes no even number higher harmonic distortion component, especially the second higher harmonic distortion component, and no lower harmonic distortion component, as shown in FIG. 6 (a). That is to say, as shown in that figure, when the intervals of zero cross points in the FM carrier are equal (the zero cross point of the basic wave carrier and the zero cross point of the input carrier coincide with each other), there is no leak component from the input carrier, which reduces the lower side band component mixed in the FM demodulation video band. In FIG. 6 (a), for convenience, the waveform of the input FM carrier is designated by a solid line and the waveform of the basic wave carrier is designated by a broken line.
On the other hand, when the input FM carrier includes the second higher harmonic distortion component, for example, as shown in FIG. 6 (b), the intervals among zero cross points are not equal (zero cross points of the basic wave carrier, zero cross points of the second higher harmonic distortion and zero cross points of the input carrier all differ), and the input leak component is increased/decreased depending on the degree to which extent the intervals between zero cross points differ from an equal interval. Also, when the center carrier F.sub.C2 which is multiplied by 2 by doubler 39 includes leak component F.sub.C ' (1/2 lower harmonic distortion component) from the input FM carrier which is not multiplied by 2 (refer to FIG. 6 (c)), similar to the above, the intervals between zero cross points are not equal, so that the leak component is increased or decreased depending on to which extent intervals between zero cross points differ from the equal interval.
The spectrum of the leak component F.sub.C ' (refer to FIG. 5 (d)) is converted as shown in FIG. 5 (f) by pulse count system FM demodulator 41, and the third lower sideband (5 MHz, the ratio with respect to leak carrier is 1.4%) of the leak component F.sub.C ' from the input carrier is present the FM demodulation video band. In addition, in pulse count system FM demodulator 41, there occur the cross modulation of upper and lower higher harmonic spectrum of the main carrier and the cross modulation of respective higher harmonics of the main carrier and the above leak component. Among them, by the cross modulation of the second lower sideband of the main carrier F.sub.C2 (14 MHz, the ratio with respect to the main carrier is 9%) and the 20% leak component F.sub.C ' (16 MHz, the ratio with respect to the main carrier is 20%), a 2 MHz component (the ratio with respect to the main carrier is 9.times.20/(2.times.100) %= 0.9%) is produced, resulting in occurrence the of beat stripes. In order to avoid the occurrence, an output of doubler 39 is supplied to a high-pass filter 40 with a cut-off frequency of 14 MHz. The leak component F.sub.C ' is attenuated by high-pass filter 40 to prevent the occurrence of beat stripes.
However, in the above-described conventional structure, the modulation index m.sub.1 is large and the spectral spread of the FM carrier spectrum into the upper/lower sidebands is large. Accordingly, when a portion of the lower sidebands of the FM carrier spectrum is cut-off by high-pass filter 40, the upper/lower sidebands are unbalanced, so that inverted white peaks are apt to be caused in an FM demodulation output. Accordingly, it is possible to prevent the inverted white peaks by reducing the spectral spread of the upper/lower sidebands by making the modulation index m.sub.1 small to suppress the frequency deviation .DELTA.F to around .+-.2 MHz. However, another problem arises in improving the SN ratio of a demodulated video signal.
Also, providing high-pass filter 40 for preventing beat stripes results in an increase in cost as well as making the structure of an apparatus more complex.